THIS invention relates to a ferrohydrostatic separation (FHS) method and apparatus.
As defined in the specification of U.S. Pat. No. 3,483,969, a ferrofluid is a material comprising a permanent, stable suspension of ferromagnetic material in a suitable liquid carrier. A common ferrofluid comprises fine particles typically 10−8M or less in size) of magnetite in a liquid. In this case, the extremely fine nature of the particles maintains them indefinitely in suspension without sinking or agglomerating.
The use of a ferrofluid to separate materials of different densities, referred to in the art as ferrohydrostatic separation (FHS), is also known and is, for instance, described in the specification of U.S. Pat. No. 3,483,969. The materials which are to be separated can be solid particulate materials or liquids which are immiscible with the carrier liquid of the ferrofluid. In essence, the separation process involves applying a magnetic field of a specific pattern to the ferrofluid with a view to controlling the apparent density of the ferrofluid within close limits. The materials which are to be separated are then deposited in the ferrofluid with the result that those materials which have a density exceeding the controlled apparent density of the ferrofluid will sink in the ferrofluid while those which have a density less than that of the ferrofluid will float in the ferrofluid. The sink and float fractions can then be recovered separately.
In all known prior art FHS separators using ferrofluids and employing electromagnets or permanent magnets with an iron yoke, the magnetic field of a specific pattern is generated in a horizontal direction with the ferrofluid situated between the pole tips of the magnet. This arrangement has the significant disadvantage that in order to achieve a magnetic field across a suitably large volume to enable the FHS technique to be used for large material throughputs, it is necessary to increase the gap between the pole tips of the magnet. This in turn results in a large and uneconomical increase in the volumes of copper and iron required to construct the magnet and, in general, in the overall size and mass of the separation apparatus. In addition, the arrangement does not lend itself to large scale-up to treat large tonnages of material.
To overcome these limitations of the conventional iron yoke-based design with a horizontally orientated magnetic field, the specification of South African patent ZA 97/9598 proposes an arrangement in which a magnetic field with specific pattern is generated in a vertical direction by means of a solenoid, typically with a ncn˜uniform winding. The use of a solenoid has numerous advantages compared to the use of an iron yoke electromagnet or permanent magnet, these being set out in the aforementioned patent specification. For instance, with a solenoid it is possible to increase the throughput merely by increasing the relevant transverse dimension of the solenoid, the axial length of the air gap remaining constant.
Although the solenoid-based proposal described in the aforementioned patent specification provides the ability to scale up the FHS technique to treat large volumes of material, the relative complexity of the winding design and of the steel cladding, together with the necessity to generate a rather high magnetic field in order to achieve the desired field pattern, are inherent disadvantages. Since a modest magnetic field strength is generally required in the FHS technique these drawbacks can, however be countered by taking advantage of the high saturation magnetisation of steel.
Another disadvantage of the conventional iron yoke FHS systems is the fact that the gradient of the magnetic field is proportional to the magnetic field strength. In order to achieve a low apparent density of the ferrofluid, for example to separate low-density materials such as coal, low magnetic field gradient and field strength are required. However the field may then be unable to retain the ferrofluid in the separation gap, necessitating complicated mechanical means to prevent the ferrofluid from running cut of the gap.